OAKLAND, Calif. (AP) — Two San Francisco Bay Area transit unions are set to go on strike and shut down one of the region’s major train systems if they don’t reach agreement on a new contract.

Bay Area Rapid Transit’s two largest unions issued a 72-hour strike notice Thursday evening. That means train service that serves more than 400,000 commuters each weekday could be shut down during the Monday morning commute if a deal isn’t reached over the weekend.

Union leaders said they issued the warning as a courtesy to riders and stressed that they intend to participate in labor talks up until the contract expires at midnight Sunday in hopes of averting a strike.

“This is not something that we want to do. This is not something that we intend to do,” said Antonette Bryant, president of the Amalgamated Transit Union Local 1555, one of two unions in talks with BART.

BART spokesman Rick Rice said the agency was disappointed by the move.

“A strike only stalls and delays the decisions that need to be made while using our riders as pawns,” he said in a statement. “BART is willing to stay at the table for as long as it takes to reach an agreement. Even if there isn’t a deal in place by Sunday night, talks can be extended.”

The two sides negotiated Thursday but did not appear close to an agreement.

Bryant said progress was made on peripheral issues, but the “meat and potatoes” issues of the contract, including salaries and benefits, had yet to be resolved.

BART, the nation’s fifth-largest rail system, carries passengers from the farthest reaches of San Francisco’s densely populated eastern suburbs across the bay, through the city, and to San Francisco International Airport.

The agency contractually cannot hire any replacement workers, but BART spokesman Jim Allison said it will secure about 95 charter buses to transport Bay Area commuters if there’s a strike.

The unions went on strike last month, shutting down BART service for four days and snarling transit in the region. Commuters faced long lines for buses and ferries, and roadways were jammed, but a transit shutdown next week could be more disruptive because the first work stoppage occurred around the Fourth of July holiday.

The unions — which represent nearly 2,400 train operators, station agents, mechanics, maintenance workers and professional staff — agreed to call off the strike and extend their contracts until Aug. 4 while negotiations continued.

Key sticking points in the labor dispute include worker safety, pensions and health care costs, according to BART and union officials.

The transit agency has said union train operators and station agents average about $71,000 in base salary and $11,000 in overtime annually. The workers pay nothing toward their pensions.

BART says it needs to save money on benefits to help pay for system improvements.

Unions submitted their last financial proposal last month and were awaiting a counteroffer from BART, said Josie Mooney, chief negotiator for the local Service Employees International Union, the other union in talks with BART. Mooney said they offered to contribute to the pension, but she could not say how much because of a gag order issued by a mediator.

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Associated Press writer Sudhin Thanawala in San Francisco contributed to this report.